Residues produced in the treatment of zinc concentrates by conventional roast-leach-electrowinning processes may contain significant quantities of zinc, but the recovery of this zinc may be hampered by the presence of interfering elements such as iron or undesirable impurities such as arsenic, germanium or fluoride. It has been well known for many years that treatment of zinc ferrite residues in a hot acid leach will solubilize the zinc. However, treatment of these residues to recover the contained zinc was not economic until the 1960's when the jarosite and goethite processes were developed for handling the iron dissolved from the ferrite residues. In certain plants, the implementation of these processes has allowed for treatment of stockpiled residues which are fed to the hot acid leach at a slow rate along with current material. Not all zinc process residues are amenable to such a treatment, however, owing to the nature of the zinc species in the residue, which may be refractory to the hot acid leach process, or to the presence of other elements, which either interfere with the recovery of zinc or must be recovered in addition to the zinc. For example, the zinc plant residue may contain significant quantities of metal values other than zinc, such as lead, copper or precious metals, but the low grade of the residue in respect of these elements may preclude their economic recovery on an individual basis.
U.S. Pat. No. 4,572,822 issued Feb. 25, 1986 to Abe and Tanaka discloses a process for recovering copper from industrial residues involving a reducing leach of the residue in sulphur dioxide atmosphere to dissolve oxidic copper compounds and the addition of elemental sulphur to precipitate copper from the solution as copper sulphide. Although this process allows for the recovery of copper found in the industrial residue in both sulphidic and oxidic forms as a single copper sulphide product, the process does not allow for the presence of zinc as zinc sulphide in the industrial residue. Zinc sulphide, if present, would report along with the copper sulphide in a subsequent step to separate the copper sulphide from the industrial residue and would render the copper sulphide product of little or no value if the zinc sulphide were present in an amount equal to or greater than the amount of the copper sulphide.
In accordance with the process of the present invention, copper sulphide product from an industrial residue is recoverable substantially free of zinc.
U.S. Pat. No. 4,676,828 issued Jun. 30, 1987 to Andre discloses a process for the extraction of zinc from zinc sulphide concentrate including leaching with a dilute aqueous solution of sulphuric acid under atmospheric pressure. This process, however, relates to the extraction of zinc and copper from sulphurous zinciferous materials, and does not permit the separation and recovery of copper and zinc from industrial residues which contain a large proportion of oxidic compounds.
The flotation of lead sulphate and the precipitation of iron from a zinc sulphate solution are discussed in Papers by Fuerstenau et al. "The Surface Characteristics and Flotation Behavior of Anglesite and Cerussite", Int. J. of Min. Proc., 1987, 20, 73-85; Andre and Masson, "The Goethite Process in Retreating Zinc Leaching Residues", presented at the 102nd AIME Annual Meeting, 1973, Chicago; Boxall and James, "Experience with the Goethite Process at National Zinc"; and in J. E. Dutrizac and A. J. Monhemius (Ed.), Iron Control in Hydrometallurgy, 1986, Ellis Horwood, Chichester, 676-686.
None of these Papers discloses or suggests the combination of steps and parameters of the process of the present invention for the recovery of metal values from industrial residues. The process of the invention permits separation of zinc, copper, lead and precious metals from zinc plant residues in a readily recoverable form and surprisingly allows for recovery of metal values present in the head material in two or more substantially different mineral forms. Zinc or copper, for example, may be recovered in high yields whether they are present as sulphidic or oxidic compounds, or both.